Guest Post: Creating with Composite Materials - Part 2 - Vacuum Bagged Wet Layup

Nov 17th 2021

In the second installment of this series, Chris Rogers from takes us through the process & some pitfalls involved with Vacuum Bagged Wet Layup.

Bagged wet layup is just like “open molding” up to the point that all the reinforcement and resin are laminated in the mold. Instead of just moving along and letting the resin cure, a vacuum bag and associated bagging consumables are applied to cover the part and vacuum is used to consolidate the laminate and remove excess resin before the resin starts to cure. Vacuum bagging is labor and material-intensive, but it can make a huge difference in laminate performance. Especially with cored laminates, the vacuum bag pressure makes for much better core bonding and helps eliminate voids in the laminate itself. You can however, overdo it and suck out too much resin!

Bagged wet layup is probably best used for low volume parts on tooling that is either not 100% vacuum tight (for infusion) or not up to high temperature service (pre-preg). For foam or balsa on contoured surfaces, bagging the core is a great idea even if the skins are open molded with only manual consolidation – aka. rolling and bubble-popping. So if you are only going to bag one thing – make it the core to the mold-side skin. For production purposes, infusion is usually a better and more consistent choice. There is just too much variability and reliance on skilled labor – and wet layup puts laminators in contact with resin for long periods of time, so it requires significant protective gear.

This process can be very tidy and can make for excellent laminate quality when carefully done. Check out Laminate Sample #15 below to see it done by wetting out the fiber on plastic – like a half-step toward pre-preg – though the resin gel time is still a limiting factor on layup time.

Here are some Laminate Samples showing bagged wet layup:

One situation where bagged wet layup is a best answer is with laminating secondary bonds to assemble components of a structure. Often parts molded separately are assembled and bonded with fillets of filled resin – then the two parts are “taped” or “tabbed” together with wet-laid material. Generally, “tabbing” implies open molded “mat, biax and polyester” secondary bonding, where “taping” implies bagged wet layup typically with epoxy. I’m sure depending on where you live and who you hang out with, there are plenty of totally different terms!